Beneath the rust-colored dunes of Oxia Planum, vast clay deposits are holding secrets from a time when Mars was a watery world, and next year the ExoMars Rosalind Franklin rover will begin drilling into them to hunt for ancient life. Scientists have discovered that these ancient clay minerals extend far beyond what was previously thought—stretching roughly 600 kilometers across and rising over a kilometer in altitude, reaching as far as the region of Mawrth Vallis, which lies some 300 kilometers from the planned landing site. The findings suggest that 4 billion years ago, a region that now looks barren and frozen may have been shaped by immense amounts of water, possibly an ocean reaching several kilometers in depth.
Clay minerals are remarkable archives of habitability. They require liquid water to form, and their very presence is a testament to a Martian climate far warmer and wetter than the planet we see today. Understanding these deposits is essential for reconstructing Mars's early climate and assessing whether the conditions ever favored life. By targeting Oxia Planum, the ExoMars mission is aiming for the oldest clays in this vast sequence, offering an unparalleled window into the planet's distant past.
The discovery came through meticulous analysis of orbital data. Scientists used the OMEGA instrument on ESA's Mars Express orbiter and the CRISM instrument on NASA's Mars Reconnaissance Orbiter to examine the mineralogy and rock layers between Oxia Planum and Mawrth Vallis. What they found was striking: both sites contain similar mineral layers, suggesting a regional—or even global—process shaped the landscape. Equally intriguing was their identification of a paleosurface, a remnant of an ancient, exposed surface that was heavily cratered and later buried by younger deposits. This paleosurface marks a crucial turning point in Martian history: a pause in sedimentation, followed by a shift in water chemistry and mineralogy across both sites. The implication is that early Mars did not experience a simple, continuous transition to dryness, but rather an intermittently wet climate with distinct periods of activity and dormancy.
"We are targeting the oldest deposits in the sequence, which makes the potential implications for the geology and early climate of Mars very relevant for the Rosalind Franklin mission in its search for life," explains Jorge Vago, ExoMars project scientist. The scale of what the rover will investigate is breathtaking. Because the clay-bearing area is so vast—stretching hundreds of kilometers—scientists are not looking at a localized accident of geology but rather a regional or global process that would have required immense amounts of water. If an ocean did indeed form these deposits, its ancient shorelines would rank among the highest ever theorized for Mars, reshaping our understanding of the planet's hydrological past.
When the Rosalind Franklin rover lands at Oxia Planum, its drill will penetrate up to two meters below the surface, far deeper than any rover before it. This capability is crucial: it will allow the mission to examine clay minerals protected from Mars's harsh surface radiation, where biosignatures—if they exist—are most likely to have been preserved. The rover will confirm what orbiters have glimpsed from above and help reconstruct a chapter of Martian history that may ultimately reveal whether the red planet was ever a home to life.